1. Field of the Invention
The present invention relates to gas generant or propellant compositions which when formed into cylindrical pellets, wafers or other appropriate physical shapes may be combusted in a suitable gas generating device to generate cool nitrogen gas and easily filterable condensed phase products. The resultant gas is then preferably used to inflate an air bag which serves as an automobile occupant cushion during a collision. More particularly this invention relates to azide-based gas generant compositions including special additives, and additive amounts, to control the linear burning rate of any such shapes produced therefrom and to control the viscosity or melting point of the slag or clinker produced.
Even though the gas generant compositions of this invention are especially designed and suited for creating nitrogen for inflating passive restraint vehicle crash bags, it is to be understood that such compositions would function equally well in other less severe inflation applications, e.g. aircraft slides, inflatable boats, and inflatable lifesaving buoy devices as in U.S. Pat. No. 4,094,028, and would in a more general sense find utility any place where a low temperature, non-toxic source of nitrogen gas is needed.
2. Description of the Prior Art
Automobile air bag systems have been developed to protect the occupant of a vehicle, in the event of a collision, by rapidly inflating a cushion or bag between the vehicle occupant and the interior of the vehicle interior.
The use of protective gas-inflated bags to cushion vehicle occupants in crash situations is now widely known and well documented. In early systems of this type, a quantity of compressed, stored gas was employed to inflate a crash bag which, when inflated, was positioned between the occupant and the windshield, steering wheel and dashboard of the vehicle. The compressed gas was released by the action of actuators or sensors which sensed a rapid change in velocity of the vehicle during a rapid impact, as would normally occur during an accident. Because of the bulk and weight of such stored, compressed gas systems, their generally slow reaction time and attendant maintenance difficulties, these type systems are now largely obsolete, having been superseded by air bag systems utilizing a gas generated by chemical gas-generating compositions. These advanced systems involve the use of an ignitable propellant composition for inflating the air cushion, wherein the inflating gas in generated by the exothermic reaction or the reactants which form the propellant.
The most common air bag systems presently in use include an on-board collision sensor, an inflator, and a collapsed, inflatable bag connected to the gas outlet of the inflator. The inflator typically has a metal housing which contains an electrically initiated igniter, a gas generant composition, for example, in pellet or tablet form, and a gas filtering system. Before it is deployed, the collapsed bag is stored behind a protective cover in the steering wheel (for a driver protection system) or in the instrument panel (for a passenger system) of the vehicle. When the sensor determines that the vehicle is involved in a collision, it sends an electrical signal to the igniter, which ignites the gas generant composition. Then the gas generant composition burns, generating a large volume of relatively cool gaseous combustion products in a very short time. The combustion products are contained and directed through the filtering system and into the bag by the inflator housing. The filtering system retains all solid and liquid combustion products within the inflator and cools the generated gas to a temperature tolerable to the vehicle passenger. The bag breaks out of its protective cover and inflates when filled with the filtered combustion products emerging from the gas outlet of the inflator. See, for example, U.S. Pat. Nos. 3,904,221 and 4,296,084.
The requirements of a gas generant suitable for use in an automobile air bag device are very demanding. The gas generant must have a burning rate such that the air bags are inflated rapidly (within approximately 30 to 100 milliseconds). The burning rate must not vary with aging or as a result of shock and vibration during normal deployment. The burning rate must also be relatively insensitive to changes in moisture content and temperature. When pressed into pellets or other solid form, the hardness and mechanical strength of the pellets must be adequate to withstand the mechanical environment to which it may be exposed without any fragmentation or change of exposed surface area. Any breakage of the pellets would potentially lead to an undesirable high pressure condition within the generator device and possible explosion.
The gas generant must efficiently produce cool, non-toxic, non-corrosive gas which is easily filtered to remove solid or liquid products, and thus preclude damage to the inflatable bag(s) or to the occupant(s) of the automobile.
The requirements as discussed in the preceding paragraphs limit the applicability of many otherwise suitable compositions from being used as air bag gas generants.
Mixtures of sodium azide and iron oxide are favored because a low reaction temperature (approximately 1000 degrees centigrade) is produced, the reaction products are solids or liquids which are easily filtered within a gas generator device, and the mixtures produce a high volume of non-toxic gas. Without the use of other oxidizers and additives, however, the burning rates are typically very low. Iron oxide is also a very hard substance which causes machinery to wear with prolonged use, and can impart a hygroscopic nature to the formulations if very fine ferric oxide is used. Some severe aging problems have also been experienced particularly when certain additives have been used in conjunction with sodium azide and ferric oxide. U.S. Pat. No. 4,203,787 discloses that ferric oxide based gas generants with azide fuels have been less preferred than other oxidizers because they burn unstably and slowly, and are difficult to compact into tablets.
The problems associated with the low burning rate of sodium azide and ferric oxide compositions have largely been overcome by the use of co-oxidizers such as an alkali metal nitrate or perchlorate (see, for example, U.S. Pat. Nos. 4,203,787; 4,547,235; 4,696,705; 4,698,107; 4,806,180 and 4,836,255. The inclusion of co-oxidizers has, however, in addition to causing an increase in the burning rate of the compositions, resulted in an increase in the flame temperature, with some consequent loss in the ability to form good solid product clinkers.
The hygroscopic nature of the sodium azide and ferric oxide formulations has been shown to be reduced by the addition of hydrophobic fumed silica (see aforementioned U.S. Pat. No. 4,836,255). The use of the hydrophobic fumed silica reduces the moisture sensitivity of sodium azide and ferric oxide compositions and also interacts with the solid or liquid products to improve clinkering by the formation of alkali metal silicates which have a higher melting point than the alkali metal oxide products. The silicates also likely serve to increase the viscosity of the liquid products making them easier to filter in a gas generator device.
The use of silicate additives for the purpose of improved clinkering and burning rate control in compositions containing sodium azide, ferric oxide, and potassium nitrate is described in aforementioned U.S. Pat. No. 4,547,235. While clinkering is improved, the large amounts of silica used were actually effective in reducing the burning rate of the formulations when the silica levels were increased at the expense of the potassium nitrate.
Aforementioned U.S. Pat. Nos. 4,696,705; 4,698,107 and 4,806,180 describe formulations comprised of sodium azide, ferric oxide, sodium nitrate, silica, bentonite (a mineral), and graphite fibers. These patents disclose the burning rate enhancement qualities of the graphite fibers, but does not expressly state the purpose and function of the bentonite and fumed silica additives. The patents also imply an equivalence of the fumed metal oxides (alumina, silica, and titania). Within these patent disclosures bentonite is not considered to be equivalent to the fumed metal oxides.
Also of interest is the teachings regarding the use of various combustion catalyts and/or slag/residue control and similar agents in azide-based propellants in general found in U.S. Pat. Nos. 2,981,616; 3,883,373; 3,947,300; 4,376,002; 4,604,151; 4,834,818 and 4,981,536.
U.S. Pat. No. 4,533,416 is also of general interest in the Example 6 teaching of adding 2% bentonite to a NaN.sub.3 --Fe.sub.2 O.sub.3 based propellant, presumably for its binding properties which proved ineffectual.
Throughout this specification all percentages of compositional ingredients are by weight based on total composition weight unless otherwise indicated.